This invention relates to a method for removing multivalent heavy metals and metal cyanide ions from metal plating waste effluents. More particularly, this invention relates to the use of unexpanded vermiculite in its native state in a cation exchange column for removal of such multivalent heavy metals and metal cyanide ions.
Metal plating is used to improve surface properties of metallic and nonmetallic products by coating a relatively thin, adherent layer of metal onto an object. Electroplating is the most common and important of the various metal plating processes. Metals commonly used in electroplating processes include nickel, copper, chromium, zinc, gold, silver, cadmium, and tin.
There are two basic types of metal plating baths used in electroplating processes. They are the simple salt (or "acid" bath) and the complex ion bath, with cyanide being the most commonly used complex ion. For example, copper can be plated from either an acid or an alkaline bath. If an acid bath is used, the following simplified explanation is typical of the plating process. Two electrodes are immersed in a copper sulfate solution and connected to a direct current electrical source. When current is applied, copper ions in solution migrate toward the negative electrode (cathode) which can be the article to be plated. The positive electrode (anode) is of copper and is the source of new copper ions in solution to replace those which are plated onto the article.
When an alkaline plating bath is used, cyanide is the anion in solution and forms a complex with the heavy metal ion to be plated. Commercial alkaline copper and zinc metal plating baths usually contain cyanide as the complexing ion; cadmium plating baths almost always use cyanide. Typical cyanide concentrations in such baths may range from 15,000 to 100,000 mg/l.
Waste water from metal plating operations amounts to several billion gallons per year in the United States alone. Buford and Mosselli, Industrial Wastes, Reinhold Publishing Corp. (1953), indicate that the major sources of these wastes are from drag-out losses carried into overflow rinses by the plated element or the rack holding the plated element, solution dumping of spent, spoiled, or obsolete solutions (rarely done unless remedial measures fail), and tank leakage losses. Although trace amounts of metals in water supplies are not harmful, their presence in greater concentrations has well known adverse and toxic effects on both plant and animal life. In addition, the cyanide ion alone and in combination with heavy metal ions is one of the most toxic of industrial wastes and is present in large quantities in many electroplating waste waters. As little as 0.05 mg/l of cyanide in water will kill many forms of aquatic life.
The United States Public Health Service has extablished drinking water standards setting the maximum acceptable concentration of such heavy metals and cyanide in water supplies. A more detailed and comprehensive listing of the toxic effects of plating wastes can be found in an Environmental Protection Agency publication entitled "Water Quality Criteria Data Book, Volume 3, Effects of Chemicals on Aquatic Life" (Pub. No. 18050 GNV 05/71). Accordingly, in view of the billions of gallons of plating wastes generated yearly, it can be seen that there is a need for an effective means to remove both the heavy metals and metal cyanide compounds from these plating wastes to avoid pollution of water supplies.
Of course, this is a recognized need, and many attempts have been made to treat such plating waste effluents. The methods used for removal of heavy metal ions have included dilution and discharge into sewers or streams; evaporation, dialysis and electrodialysis; reverse osmosis; and land disposal. However, the two most common methods of heavy metal ion removal are neutralization-precipitation and ion exchange coupled with neutralization-precipitation. With respect to the cyanide portion of the plating waste effluents, treatments have included alkaline chlorination, acidification with volatilization and recovery, biological treatment, complexing, dilution, electrolytic oxidation, thermal decomposition, ion exchange, ozonation, and peroxygen compound treatment. Alkaline chlorination is by far the most common method of cyanide removal used today.
If cyanide is present in the metal plating wastes, it is almost always found in a complex with heavy metal ions. Since it is next to impossible to remove the heavy metal ions from the waste stream without first destroying the cyanide complex, it can be seen that at least a two-step removal process is necessitated when cyanide ion is present.
However, most, if not all, of the prior processes suffer from shortcomings. When alkaline chlorination is used as the process to destroy the cyanide complex, high amounts of chloride ion become dissolved in the waste and residuals of unreacted chloride can be very toxic to aquatic life. Equipment to safely contain these corrosive and toxic chemicals is costly. The Kastone process (Kastone is a registered trademark of E. I. duPont de Nemours & Co.) utilizes hydrogen peroxide and formaldehyde to convert cyanide ion to the less toxic cyanate ion followed by acidification to hydrolyze the cyanate ion to ammonium ion and carbon dioxide. The heavy metals present in the treated stream of the Kastone process are precipitated usually as oxides or hydroxides. However, heavy metal flocs (hydroxides) are themselves difficult to remove from a waste stream since they are bound with and have approximately the same density as water. Separation of precipitated metal hydroxides is both cumbersome and expensive involving further addition of thickening and settling aids and treatment with sludge thickening and drying apparatus, vacuum filtration, and/or centrifugal dewatering.
Ion exchange has also been utilized to concentrate both cyanide-heavy metal complexes and heavy metal ions in plating waste effluents to facilitate their later removal or recovery. An important advantage of ion exchange treatment is the savings of water due to recirculation of treated water. However, past methods using ion exchange have also suffered shortcomings including the presence of impurities in the waste which are destructive to ion exchange resins, the presence of interfering ions, a limited loading capacity of ion exchange columns, and relatively high operating costs. Even after regeneration, the metal ion precipitated from the spent regenerant solution constitutes a sludge that is difficult to dispose of in an acceptable manner. A discussion of recent literature involving the use of ion exchange resins in removing heavy metal ions from metal plating wastes may be found in Etzel, U.S. Pat. No. 4,100,065 to which reference is made. As can be seen, the need still exists for an efficient, safe, and relatively inexpensive method of removing heavy metal and cyanide ions from plating wastes.